1. Field of the Invention
This invention relates to an image data generating apparatus, an image data generating method and a printing system for printing with a printing machine.
2. Description of the Related Art
Conventionally, a printing system is known in which a digital printing machine or the like prints, on a printing medium, data of output images for printing written in a page description language, through RIP (Raster Image Processor) processing which converts the data into raster data printable by the printing machine. As an output machine of such a printing system, an inkjet printing machine is known, which carries out printing by moving a printing medium relative to a recording head with a plurality of nozzles arranged thereon for discharging minute droplets of ink.
When printing with such an inkjet printing machine, in order to reduce density variations of the ink due to individual differences of the plurality of nozzles, print density is uniformed (which is called shading correction) using a plurality of correction values with respect to the plurality of nozzles (see Japanese Unexamined Patent Publication No. 2013-169756).
The number of droplet sizes of the ink discharged from the nozzles of the inkjet printing machine is limited (eg three—such as large, medium and small). In order to express density variations of color by dot sizes in print, a halftone dot process is carried out to create halftone image data by applying a plurality of threshold value matrices (SPM: Screen Pattern Memories) to image data. In carrying out such a halftone dot process, the printing machine is provided with a plurality of matrix sets which are combinations of the threshold value matrices, and what is called multi-SPM is employed for switching the matrix sets for each image according to the contents (natural pictures, characters, and so on) to be expressed (see Japanese Unexamined Patent Publication No. 2010-221584).
The number of nozzles becomes enormous in what is called a one-pass type inkjet printing machine which includes a recording head with ink-discharge nozzles arranged over the width of printing paper, thereby to be able to perform printing by passing the printing paper under the recording head once. In the one-pass type inkjet printing machine, therefore, the shading correction is done on the printing machine side. And the halftone dot process is executed after the shading correction is carried out on the image data having undergone the RIP processing. In order to realize multi-SPM for such an inkjet printing machine, it has been necessary to generate and supply the printing machine with data independent of the image data for communicating SPM selection information to a processor which executes the halftone dot process. Conventionally, therefore, the load of transmitting data to the printing machine has increased by an amount corresponding to the data for multi-SPM.
The increase of the load of transmitting data to the printing machine may reduce operation efficiency of the printing machine in a printing system which performs high-speed printing. While it is conceivable to increase the data transmission capacity of the apparatus in order to cope with such increase of the load of transmitting data to the printing machine, the apparatus will become expensive in that case.